Such optical recording disk drives have all been expanded in capacity and CD drives using conventional infrared laser beams (wavelength: 780 nm), DVD drives using red laser beams (wavelength: 650 nm), and blu-ray disk drives (BD) and high density (HD) DVD drives using blue laser beams are already under manufacturing. In optical disk drives such as optical disk reading/recording drives as described above, each high frequency signal (read RF signals) read from an optical disk includes so-called laser noise generated from the beam irradiated on the optical disk. The characteristic of this laser noise do not differ so much between LSs of CD (wavelength: 780 nm) and DVD (wavelength: 650 nm), since compound semiconductor materials used for those disks, as well as their LD wavelengths are almost the same. In the case of the blue laser used for the next generation DVD drives, however, the characteristic of the LD comes to be different from that of the conventional ones, since a special compound semiconductor material referred to as GaN is used for the blue laser.
For example, the laser noise characteristic is represented as RIN (Relative Intensity Noise). According to the BD standard, this RIN is ruled as −125 dB/Hz or under. FIG. 2 shows a relationship between system noise and laser noise. The system noise is composed of all types of noise such as laser noise, amplification noise, etc. generated from the subject system. At present, the BD employs a one-time speed. In case the speed is improved to a 2-time speed or 4-time speed in the future, however, such system noise is required to be reduced and the main system noise, that is, laser noise is required to be further reduced as the operation of the BD is speeded up more and more.
When writing data into an optical disk drive, the required output value is 30 mW or over at present in case a blue laser is used. On the contrary, the blue laser power on the surface of the disk when in reading is ruled as 0.35 mW±0.1 by the BD standard. And, in order to satisfy the requirement, the output blue laser power when in reading is required to be 2 mW or under. The conventional blue laser cannot satisfy both of the requirements, that is, large output power and less noise generation with any ordinary methods that use the BD respectively.
In order to realize highly accurate data reading, there have been proposed some methods for removing laser noise from read RF signals. For example, the patent document 1 (JP-A No. 183970/2002) discloses a first multiplier 16-1 shown in FIG. 3, in which a read RF signal rf (t) is multiplied by a DC component of an APC monitor output signal m (t) output from an LPF 15 and the signal that is a result of the multiplication is supplied to a calculator 17. In a second multiplier 16-2, a read RF signal rf(t) is multiplied by a laser noise component of the APC monitor output signal m output from an HPF 18 and the signal that is a result of the multiplication is supplied to the calculator 17. And, in the calculator 17, the signal output from the multiplier 16-2 is reduced from the signal output from the multiplier 16-1 to remove both of the laser noise additive noise component and the modulation noise component. Each optical disk drive provided with a reading mechanism is designed to realize desirable reading.
The patent document 2 (JP-A No. 229636/2003) discloses a method that forms an attenuator for adjusting an optical output of the laser beam in a semiconductor laser element that includes a laser beam source for irradiating a laser beam onto a semiconductor substrate and a modulator for modulating the laser beam in the same laser structure as that of the laser beam source.